Anonymous crowd comparison

ABSTRACT

Systems and methods are disclosed for anonymously comparing user groups, such as but not limited to crowds, to determine a degree of user overlap. In general, a hash value is obtained for a first user group, where the hash value includes a hash value component for a number of two-user permutations within the first user group. Similarly, a hash value is obtained for a second user group, where the hash value includes a hash value component for a number of two-user permutations within the second user group. Thereafter, a degree of user overlap between the first and second user groups is determined based on a comparison of the hash value for the first user group and the hash value for the second user group.

RELATED APPLICATIONS

This application claims the benefit of provisional patent application Ser. No. 61/173,625, filed Apr. 29, 2009, the disclosure of which is hereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to comparing user groups, such as crowds of users, to determine a degree of user overlap between the user groups in a manner that preserves privacy.

BACKGROUND

A mobile user can benefit from knowing how many people in his or her current crowd have been in previous crowds with the mobile user or how many people in his or her current crowd have frequented the mobile user's current location in the past. A simple mechanism to implement this functionality would require tracking the locations of people over time. However, this approach does not respect user privacy. As such, there is a need for a system and method that enables crowd comparison in a manner that maintains user privacy.

SUMMARY

Systems and methods are disclosed for anonymously comparing user groups, such as but not limited to crowds, to determine a degree of user overlap. In general, a hash value is obtained for a first user group, where the hash value includes a hash value component for a number of two-user permutations within the first user group. Similarly, a hash value is obtained for a second user group, where the hash value includes a hash value component for a number of two-user permutations within the second user group. Thereafter, a degree of user overlap between the first and second user groups is determined based on a comparison of the hash value for the first user group and the hash value for the second user group. More specifically, in one embodiment, the hash value for the first user group is compared to the hash value for the second user group to determine a number of matching component hash values. A number of matching users in the first and second user groups is then determined based on the number of matching component hash values.

Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention.

FIG. 1 illustrates a process for anonymously comparing two user groups based on hash values obtained for the two user groups according to one embodiment of the present disclosure;

FIG. 2 illustrates a process for computing a hash value for a user group according to one embodiment of the present disclosure;

FIGS. 3A through 3F graphically illustrate the process of FIG. 2 for an exemplary user group according to one embodiment of the present disclosure;

FIG. 4 illustrates a process for determining a degree of user overlap between two user groups based on a comparison of hash values for the two user groups according to one embodiment of the present disclosure;

FIG. 5 illustrates an exemplary system for providing anonymous crowd comparison according to one embodiment of the present disclosure;

FIG. 6 illustrates the operation of the crowd server of FIG. 5 to compute and store a crowd hash value for a crowd of interest of a requestor according to one embodiment of the present disclosure;

FIG. 7 illustrates the operation of the crowd server of FIG. 5 to compare a current crowd to one or more previously identified crowds of interest of a requestor according to one embodiment of the present disclosure;

FIG. 8 illustrates the operation of the crowd server of FIG. 5 to provide Point of Interest (POI) recommendations to a requestor based on comparisons of current crowds at POIs and one or more previously identified crowds of interest of the requestor according to one embodiment of the present disclosure;

FIG. 9 illustrates the operation of the crowd server of FIG. 5 to compare one or more historical crowds to one or more previously identified crowds of interest of a requestor according to one embodiment of the present disclosure;

FIG. 10 illustrates the operation of the crowd server of FIG. 5 to provide POI recommendations to a requestor based on comparisons of historical crowds at POIs and one or more previously identified crowds of interest of the requestor according to one embodiment of the present disclosure;

FIG. 11 illustrates the operation of the crowd server of FIG. 5 to characterize crowd patterns at a POI according to one embodiment of the present disclosure;

FIG. 12 is a block diagram of the crowd server of FIG. 5 according to one embodiment of the present disclosure;

FIG. 13 is a block diagram of one of the mobile devices of FIG. 5 according to one embodiment of the present disclosure;

FIG. 14 is a block diagram of the subscriber device of FIG. 5 according to one embodiment of the present disclosure; and

FIG. 15 is a block diagram of a computing device, such as a server, hosting the third-party service of FIG. 5 according to one embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

FIG. 1 illustrates a process for anonymously comparing two user groups to determine a degree of user overlap according to one embodiment of the present disclosure. As used herein, a user group is any group of associated users. For example, a user group may be a crowd of users that are spatially proximate one another, a distributed group of users such as users participating in a communication session (e.g., a conference call or a group chat session), or the like.

First, a hash value is obtained for a first user group (step 100). The hash value for the first user group may be obtained by computing the hash value for the first user group or by obtaining a previously computed hash value for the first user group from storage. The hash value for the first user group includes component hash values generated for a number of distinct two-user permutations for the first user group. As user herein, a “distinct two-user permutation” is a subset of two distinct users in a user group where ordering of users does not matter (i.e., a two-user permutation for users A and B is the same as, or not distinct from, a two-user permutation for users B and A). Therefore, for example, if the first user group includes user A, user B, and user C, the distinct two-user permutations for the first user group are user A and user B, user A and user C, and user B and user C. As described below in detail, the distinct two-user permutations for the first user group for which component hash values are generated and included in the hash value for the first user group may be all distinct two-user permutations for the first user group or all distinct two-user permutations for the first user group other than those including a requesting user from the first user group that initiated a process for generating the hash value for the first user group.

In a similar manner, a hash value is obtained for a second user group (step 102). The hash value for the second user group may be obtained by computing the hash value for the second user group or by obtaining a previously computed hash value for the second user group from storage. The hash value for the second user group includes component hash values generated for a number of distinct two-user permutations for the second user group. As described below in detail, the distinct two-user permutations for the second user group for which component hash values are generated and included in the hash value for the second user group may be all distinct two-user permutations for the second user group or all distinct two-user permutations for the second user group other than those including a requesting user from the second user group that initiated a process for generating the hash value for the second user group. It should be noted that while two-user permutations are used herein, permutations of three or more users may alternatively be used to generate the component hash values of the hash values for the first and second user groups.

Next, a degree of user overlap between the first and second user groups is determined based on a comparison of the hash values for the first and second user groups (step 104). As described below in more detail, the component hash values for the first and second user groups are compared to determine a number of matching component hash values in the hash values for the first and second user groups. Then, a number of matching users in the first and second user groups is determined based on the number of matching component hash values. The degree of user overlap between the first and second user groups may then be provided as the number of matching users, a percentage of matching users for a larger of the first and second user groups, or the like. In this embodiment, the degree of user overlap between the first and second user groups is then output to a requesting entity (step 106). The requesting entity may be a user, a third-party service, or the like. In addition or alternatively, the degree of user overlap between the first and second user groups may be stored for subsequent use in any desired application.

FIG. 2 illustrates a process for computing a hash value for a user group according to one embodiment of the present disclosure. First, a list of users in the user group is obtained (step 200). The list of users includes a user identifier (ID) for each user in the user group. The user IDs of the users in the user group may be, for example, names of the users, usernames or screen names of the users, e-mail addresses of the users, telephone numbers of the users, or the like. Next, a requesting user, or requestor, is removed from the list of users, if applicable (step 202). The requesting user is a user in the user group that initiated generation of the hash value for the user group. Removing the requesting user from the list of users for the user group results in different hash values for the same user group for different requestors. This enhances security by preventing stored hash values for a first requestor to be directly compared to stored hash values for a second requestor to determine whether the first and second requestors are or have been in the same user groups. Note that step 202 is optional.

Next, the list of users for the user group is sorted (step 204). In one embodiment, the list of users is sorted alphabetically. However, the present disclosure is not limited thereto. Sorting the list of users ensures that the two-user permutations are canonical between user group hash instances. In other words, if two users A and B exist in a user group, the two-user permutation for users A and B will always be “A B” and not “B A.” This ensures that a component hash value for a two-user permutation for users A and B in a hash value for one user group will match a component hash value for a two-user permutation for the same two users in a hash value for another user group.

Next, a list of all distinct two-user permutations for the sorted list of users for the user group is created (step 206). A next two-user permutation from the list of distinct two-user permutations is then obtained (step 208). Note that for the first iteration, the next two-user permutation is the first two-user permutation in the list of distinct two-user permutations. A hash value (referred to herein as a component hash value) is then computed for the two-user permutation using a predetermined hash function (step 210). The predetermined hash function may be, for example, Secure Hash Algorithm-1 (SHA-1), but is not limited thereto. The component hash value for the two-user permutation may be computed by concatenating the user IDs of the users in the two-user permutation and providing the concatenated user IDs as an input to the predetermined hash function. The hash value returned by the predetermined hash function is then used as the component hash value for the two-user permutation.

A determination is then made as to whether the last two-user permutation has been processed (step 212). If not, the process returns to step 208 and is repeated. Once component hash values have been computed for all of the two-user permutations from step 206, the component hash values for the two-user permutations are concatenated to provide a concatenated hash value for the user group (step 214). In this embodiment, the concatenated hash value for the user group is compressed using a lossy compression algorithm to provide a hash value for the user group (step 216). In one embodiment, the lossy compression algorithm removes every Nth bit from the concatenated hash value to provide the hash value for the user group, where N is greater than or equal to 2. In one preferred embodiment, the lossy compression algorithm removes every other bit from the concatenated hash value to provide the hash value for the user group. Lossy compression reduces the storage space needed to store the hash value for the user group. In addition, lossy compression further obfuscates user information by making it highly improbable that the system or an attacker can retrieve the exact concatenated hash value or exact component hash values. In other words, lossy compression thwarts dictionary attacks. Note that the aforementioned embodiments of the lossy compression algorithm are exemplary. Other types of lossy compression algorithms may be used, as will be appreciated by one of ordinary skill in the art upon reading this disclosure. Also note that step 216 is optional.

FIGS. 3A through 3F graphically illustrate the process of FIG. 2 for an exemplary user group. FIG. 3A illustrates a list of users for the user group. In this example, the list of users includes users C, B, A, and D. In this example, user A is the requesting user that initiated generation of a hash value for the user group. FIG. 3B illustrates the list of users for the user group after removal of the requestor (user A) and sorting. Next, as illustrated in FIG. 3C, two-user permutations are created for the list of users from FIG. 3B. In this example, the two-user permutations include a two-user permutation for users B and C, a two-user permutation for users B and D, and a two-user permutation for users C and D. Then, as illustrated in FIGS. 3D and 3E, component hash values are computed for the two-user permutations and concatenated to provide a concatenated hash value for the user group. The concatenated hash value is then compressed using a lossy compression algorithm to provide a hash value for the user group, as illustrated in FIG. 3F.

FIG. 4 illustrates a process for determining a degree of user overlap between two user groups by comparing hash values generated for the two user groups using the process of FIG. 2 according to one embodiment of the present disclosure. First, a counter is initialized to zero (step 300). Then, a next component hash value is obtained from the hash value for the first user group (step 302). Note that, for the first iteration, the next component hash value is the first component hash value. In one embodiment, the hash function used to generate the component hash values provides 160-bit hash values and lossy compression removes every other bit such that, after lossy compression, each component hash value is 80 bits. As such, the next component hash value is the next 80 bits of the hash value for the first user group. Likewise, a next component hash value is then obtained from the hash value for the second user group (step 304). Note that, for the first iteration, the next component hash value from the hash value for the second user group is the first component hash value from the hash value for the second user group.

Next, the component hash value from the hash value for the first user group is compared to the component hash value from the hash value for the second user group to determine whether there is a match (step 306). Here, the component hash values match if they are exactly the same. Matching hash values indicate that the corresponding two-user permutations from the first and second user groups also match. If the component hash values match, the counter is incremented (step 308) and then the process proceeds to step 312. If the component hash values do not match, a determination is made as to whether the last component hash value from the hash value for the second user group has been processed (step 310). If not, the process returns to step 304 and is repeated for the next component hash value from the hash value for the second user group. If the last component hash value from the hash value for the second user group has been processed, the process proceeds to step 312.

At this point, whether proceeding from step 308 or step 310, a determination is made as to whether the last component hash value from the hash value for the first user group has been processed (step 312). If not, the process returns to step 302 and is repeated for the next component hash value from the hash value for the first user group. Once all of the component hash values from the hash value for the first user group have been processed, the counter corresponds to a number of matching component hash values, which is also the number of matching two-user permutations for the first and second user groups.

Next, a number of matching users in the first and second user groups is determined based on the number of matching two-user permutations (step 314). In the preferred embodiment, the number of matching users is determined based on the following choose-2 function:

${C\left( {n,2} \right)} = {\frac{n!}{{2!}{\left( {n - 2} \right)!}}.}$

More specifically, the number of matching users is determined by setting the choose-2 function C(n,2) equal to the number of matching two-user permutations (i.e., the counter value) as follows:

$\begin{matrix} {{C\left( {n,2} \right)} = \frac{n!}{{2!}{\left( {n - 1} \right)!}}} \\ {{= {{number\_ of}{\_ matching}{\_ two}{\_ user}{\_ permutations}}},} \end{matrix}$

where the value n can be solved for or otherwise determined and corresponds to the number of matching users for the first and second user groups. As such, if for example the number of matching two-user permutations is 6, then the number of matching users is determined based on the equation:

${6 = \frac{n!}{{2!}{\left( {n - 2} \right)!}}},$

which results in

-   -   n=4,         where n is the number of matching users in the first and second         user groups. Note that the value of n in the choose-2 function         may be mathematically solved. As one exemplary alternative, the         value of n may be determined using a lookup table populated         using the choose-2 function to enable lookup of the number of         matching users for different numbers of matching two-user         permutations values (i.e., 1, 2, 3, 4, 5, . . . , M, where M is         a desired positive integer value).

In addition to determining the number of matching users, in this embodiment, a number of users in a largest of the first and second user groups is determined (step 316). More specifically, in one embodiment, a number of two-user permutations for the first and second user groups are determined. For example, if the component hash values are 80 bit values, the number of two-user permutations in the first user group may be determined by dividing the number of bits in the hash value for the first user group by 80. Likewise, the number of two-user permutations in the second user group may be determined by dividing the number of bits in the hash value for the second user group by 80. The user group having the largest number of two-user permutations is identified as the largest of the first and second user groups. Then, the number of users in the largest user group is determined based on the choose-2 function, where the choose-2 function is set equal to the number of two-user permutations in the largest user group. For example, if the number of two-user permutations in the largest user group is 15, then the number of users in the largest user group is 6 (i.e., C(n,2)=15, therefore n=6). A percentage of matching users for the largest user group is then computed (step 318). The percentage of matching users for the largest user group may be computed based on the following equation:

${\% \mspace{14mu} {MatchingUsers}} = {\frac{{number\_ of}{\_ matching}{\_ users}}{{number\_ of}{\_ users}{\_ in}{\_ largest}{\_ user}{\_ group}} \times 100.}$

The percentage of matching users may then be output and/or stored as the degree of user overlap for the first and second user groups. Note that the percentage of matching users is exemplary and is not intended to limit the scope of the present disclosure. The degree of user overlap may be expressed as any value that is a function of (including being equal to) the number of matching users between the first and second user groups.

FIG. 5 illustrates an exemplary system 10 that operates to anonymously compare user groups according to one embodiment of the present disclosure. In the system 10, the user groups are crowds formed or otherwise determined by a crowd server 12. As illustrated, the system 10 includes the crowd server 12, a number of mobile devices 14-1 through 14-N having associated users 16-1 through 16-N, a subscriber device 18 having an associated subscriber 20, and a third-party service 22 communicatively coupled via a network 24. The network 24 may be a Local Area Network (LAN), Wide

Area Network (WAN), or the like, and may include wired and/or wireless components. In one embodiment, the network 24 is a distributed public network such as the Internet.

The crowd server 12 includes a crowd formation function 26, a crowd hash computing function 28, a crowd comparison function 30, and a crowd hash repository 32. The crowd formation function 26, the crowd hash computing function 28, and the crowd comparison function 30 are preferably implemented in software, but are not limited thereto. The crowd formation function 26 generally operates to form and possibly maintain crowds of users. More specifically, in one embodiment, the mobile devices 14-1 through 14-N provide location updates that define current locations of the users 16-1 through 16-N over time. The location updates may be provided from the mobile devices 14-1 through 14-N directly to the crowd server 12 or provided from the mobile devices 14-1 through 14-N to a location service that then reports the location updates to the crowd server 12 or otherwise enables the crowd server 12 to access the location updates. Using the current locations of the users 16-1 through 16-N, the crowd formation function 26 performs a spatial crowd formation process to form crowds of the users 16-1 through 16-N that are spatially proximate to one another. While not essential, for more detailed information regarding exemplary spatial crowd formation processes, the interested reader is directed to U.S. patent application Ser. No. 12/645,535 entitled MAINTAINING A HISTORICAL RECORD OF ANONYMIZED USER PROFILE DATA BY LOCATION FOR USERS IN A MOBILE ENVIRONMENT, U.S. patent application Ser. No. 12/645,532 entitled FORMING CROWDS AND PROVIDING ACCESS TO CROWD DATA IN A MOBILE ENVIRONMENT, U.S. patent application Ser. No. 12/645,539 entitled ANONYMOUS CROWD TRACKING, U.S. patent application Ser. No. 12/645,544 entitled MODIFYING A USER'S CONTRIBUTION TO AN AGGREGATE PROFILE BASED ON TIME BETWEEN LOCATION UPDATES AND EXTERNAL EVENTS, U.S. patent application Ser. No. 12/645,546 entitled CROWD FORMATION FOR MOBILE DEVICE USERS, U.S. patent application Ser. No. 12/645,556 entitled SERVING A REQUEST FOR DATA FROM A HISTORICAL RECORD OF ANONYMIZED USER PROFILE DATA IN A MOBILE ENVIRONMENT, and U.S. patent application Ser. No. 12/645,560 entitled HANDLING CROWD REQUESTS FOR LARGE GEOGRAPHIC AREAS, all of which were filed on Dec. 23, 2009 and are hereby incorporated herein by reference in their entireties.

The crowd hash computing function 28 generally operates to compute hash values for crowds of users formed by the crowd formation function 26. The crowd hash computing function 28 preferably utilizes the process of FIG. 2 to compute a hash value (i.e., a crowd hash value) for a crowd. The crowd hash values for at least some crowds are stored in the crowd hash repository 32. For instance, in one embodiment, the crowd hash repository 32 stores crowd hash values for crowds of interest to the users 16-1 through 16-N. Using the user 16-1 as an example, the user 16-1 may determine that a current crowd of users in which the user 16-1 is currently located is a crowd of interest (e.g., a favorite crowd) and manually initiate computation and storage of a crowd hash value for that crowd. The crowd hash for the crowd of interest to the user 16-1 may then be computed by the crowd hash computing function 28 and be stored in the crowd hash repository 32 as a crowd of interest (e.g., a favorite crowd) of the user 16-1. The crowd hash repository 32 may additionally or alternatively store crowd hash values for currently formed crowds (referred to herein as current crowds) and/or previously formed crowds (referred to herein as historical crowds). The crowd comparison function 30 operates to anonymously compare crowds formed by the crowd formation function 26 by comparing corresponding crowd hash values. More specifically, the crowd comparison function 30 compares crowd hash values for crowds to determine a degree of user overlap between the crowds.

In the preferred embodiment, the crowd server 12 only stores the current locations of the users 16-1 through 16-N, rather than a historical record of the locations of the users 16-1 through 16-N. However, the present disclosure is not limited thereto. Storage of crowd hash values for crowds enables comparison of historical crowds to one another or to a current crowd without the need to store historical records of the locations of the users 16-1 through 16-N. In this manner, privacy of the users 16-1 through 16-N is preserved. In addition, the anonymous crowd comparison described herein enables a requestor to determine the degree of user overlap between two crowds but not the identities (or user IDs) of the users in those crowds.

The mobile devices 14-1 through 14-N are mobile devices such as, for example, mobile smart phones (e.g., Apple® iPhone), laptop or notebook computers, tablet computers (e.g., Apple® iPad), or the like. The mobile devices 14-1 through 14-N include crowd clients 34-1 through 34-N and location determination functions 36-1 through 36-N. Using the mobile device 14-1 as an example, the crowd client 34-1 is preferably implemented in software, but is not limited thereto. The crowd client 34-1 generally operates to interact with the crowd server 12. More specifically, the crowd client 34-1 operates to obtain the current location of the mobile device 14-1 from the location determination function 36-1 and send corresponding location updates to the crowd server 12, either directly or indirectly. In addition, the crowd client 34-1 preferably enables the user 16-1 to interact with the crowd server 12 to initiate storage of crowd hash values for crowds of interest to the user 16-1. Still further, the crowd client 34-1 may enable the user 16-1 to initiate anonymous crowd comparison of the crowds of interest for which corresponding crowd hash values were previously computed and stored to either current or historical crowds. The crowd client 34-1 may additionally or alternatively enable the user 16-1 to request and receive Point of Interest (POI) recommendations based on anonymous crowd comparisons of the crowds of interest for which corresponding crowd hash values were previously computed and stored to either current or historical crowds at POIs within a desired geographic region.

The location determination function 36-1 may generally be any software and/or hardware component enabled to determine the current location of the mobile device 14-1. For example, the location determination function 36-1 may be a Global Positioning System (GPS) receiver.

The subscriber device 18 may be any type of user device such as, for example, a personal computer, a mobile smart phone, a laptop or notebook computer, a tablet computer, or the like. In general, the subscriber device 18 enables the subscriber 20 to access the crowd server 12 preferably for a subscription fee. In this embodiment, the subscriber 20 is enabled to access the crowd server 12 via a web browser 38 and corresponding web interface of the crowd server 12. However, rather than the web browser 38, a custom application may be used to access the crowd server 12. Via the crowd server 12, the subscriber 20 may be enabled to monitor crowd patterns at one or more desired POIs.

The third-party service 22 may generally be any type of service that desires information regarding crowds of users. For example, the third-party service 22 may be a targeted advertising service. Via the crowd server 12, the third-party service 22 may be enabled to monitor crowd patterns at one or more desired POIs and then utilize those crowd patterns to provide a desired service.

FIG. 6 illustrates the operation of the crowd server 12 of FIG. 5 to compute and store crowd hash values for crowds of interest to users. Note that while this discussion focuses on computing and storing a crowd hash value for a crowd of interest to the user 16-1, the crowd server 12 may similarly compute and store crowd hash values for crowds of interest to the other users 16-2 through 16-N. Further, in this example, the crowds of interest are favorite crowds. First, the mobile device 14-1 sends a request to store the user's current crowd as a favorite crowd to the crowd server 12 (step 400). In this embodiment, the request is initiated by the user 16-1 when the user 16-1 is currently in a crowd that the user 16-1 deems to be a favorite crowd (i.e., a crowd that the user 16-1 likes). However, the user 16-1 may additionally or alternatively initiate a request when the user 16-1 is in a crowd that the user 16-1 deems to be a crowd of interest (e.g., a crowd that he likes or a crowd that he dislikes). Also, while in this embodiment the request is initiated by the user 16-1 manually, the present disclosure is not limited thereto. The request may alternatively be initiated by the crowd client 34-1 of the mobile device 14-1 automatically in response to a predefined triggering event.

In response to receiving the request, the crowd hash computing function 28 of the crowd server 12 obtains a current crowd in which the user 16-1 of the mobile device 14-1 is currently located from the crowd formation function 26 (step 402). In one embodiment, the crowd formation function 26 forms the current crowd of the user 16-1 reactively in response to the request. Alternatively, the crowd formation function 26 may proactively form crowds and store corresponding crowd records that identify the users that are currently in those crowds. The crowd formation function 26 may then obtain the crowd in which the user 16-1 is currently located from storage.

Next, the crowd hash computing function 28 of the crowd server 12 computes a crowd hash for the current crowd of the user 16-1 using the process of FIG. 2 (step 404). More specifically, in one embodiment, the crowd hash computing function 28 removes the user 16-1 from a list of users in the current crowd, sorts the list of users, and then creates all distinct two-user permutations for the sorted list of users. The crowd hash computing function 28 then computes a component hash value for each two-user permutation and concatenates the component hash values for the two-user permutations to provide a concatenated hash value for the current crowd. Lastly, the crowd hash computing function 28 compresses the concatenated hash value for the current crowd using a lossy compression algorithm to provide the crowd hash for the current crowd.

The crowd hash computing function 28 then stores the crowd hash for the current crowd in the crowd hash repository 32 as a crowd hash value of a favorite crowd of the user 16-1 (step 406). In one embodiment, the crowd hash value is stored in a crowd hash record that includes a record ID field that stores an ID for the crowd hash record, an owner field that identifies the user 16-1 as the owner of the crowd hash record, and a hash value field that stores the crowd hash value of the current crowd. In addition, the crowd hash record may include a description field that stores a textual description of the current crowd for which the crowd hash value was computed, where the textual description may be provided by the user 16-1 (e.g., “Cool Crowd at Night Club X”). Still further, the crowd hash record may include a time field that stores a time at which the crowd hash value was computed and latitude and longitude fields that store a location of the current crowd for which the crowd hash was computed. The location of the current crowd may be computed using the current locations of the users in the crowd and, for example, a center of mass algorithm. Note that in order to thwart attacks on the system 10, in one embodiment, the crowd hash records stored by the crowd server 12 in the crowd hash repository 32 do not include any information (e.g., location, time, etc.) that may enable the system 10 or an attacker to directly correlate two crowd hash values generated for two different requestors to determine that those two different requestors were in the same crowd. Steps 400 through 406 may subsequently be repeated to compute and store crowd hash values for additional favorite crowds of the user 16-1.

FIG. 7 illustrates the operation of the crowd server 12 to anonymously compare a current crowd to one or more previously identified crowds of interest (e.g., favorite crowds) of a user according to one embodiment of the present disclosure. First, the mobile device 14-1, and particularly the crowd client 34-1, sends a current crowd comparison request to the crowd server 12 (step 500). In response, the crowd comparison function 30 obtains a current crowd in which the user 16-1 is located from the crowd formation function 26 (step 502). The crowd comparison function 30 then computes a crowd hash for the current crowd of the user 16-1 using the process of FIG. 2 (step 504). Next, the crowd comparison function 30 determines a degree of user overlap between the current crowd of the user 16-1 and one or more favorite crowds previously identified by the user 16-1 using the process of FIG. 4 (step 506). More specifically, for each of the favorite crowds of the user 16-1, the crowd comparison function 30 determines a degree of user overlap between the current crowd of the user 16-1 and the favorite crowd of the user 16-1 based on a comparison of the crowd hash value of the current crowd computed in step 504 and a crowd hash value stored for the favorite crowd in the crowd hash repository 32. The crowd comparison function 30 then returns the degree of overlap between the current crowd and each of the favorite crowds of the user 16-1 to the mobile device 14-1 (step 508). In response, the crowd client 34-1 of the mobile device 14-1 presents the degree of overlap between the current crowd and each of the favorite crowds of the user 16-1 to the user 16-1 (step 510).

Note that while the process of FIG. 7 compares the current crowd in which the user 16-1 is located to the one or more previously identified favorite crowds of the user 16-1, the present disclosure is not limited thereto. More generally, any current crowd that is relevant to the current crowd comparison request may be compared to the previously identified crowd(s) of interest of the user 16-1. The relevant current crowd may be the current crowd in which the user 16-1 is located (as in FIG. 7), a current crowd at a POI identified in the current crowd comparison request, a current crowd in a geographic region (e.g., an Area of Interest) identified by the current crowd comparison request, or the like.

FIG. 8 illustrates the operation of the crowd server 12 to provide POI recommendations based on anonymous crowd comparisons according to one embodiment of the present disclosure. First, the mobile device 14-1, and more particularly the crowd client 34-1, sends a POI recommendation request to the crowd server 12 (step 600). The POI recommendation request may be manually initiated by the user 16-1 or automatically initiated by the crowd client 34-1 in response to a predefined triggering event such as, for example, detecting an Internet search query from the user 16-1 for nearby businesses.

In response to receiving the POI recommendation request, the crowd hash computing function 28 obtains a list of relevant POIs (step 602). The list of relevant POIs may include POIs within a defined geographic region such as, for example, a defined geographic region in which the user 16-1 is currently located (e.g., within 10 miles from the current location of the user 16-1) or a geographic region defined by the user 16-1 and included in the POI recommendation request (e.g., Raleigh, N.C.). The crowd hash computing function 28 then obtains current crowds at the relevant POIs from the crowd formation function 26 (step 604).

Next, the crowd hash computing function 28 computes a crowd hash value for each of the current crowds at the relevant POIs using the process of FIG. 2 (step 606). Then, for each current crowd, the crowd comparison function 30 determines a degree of user overlap between the current crowd and one or more favorite crowds previously identified by the user 16-1 using the process of FIG. 4 (step 608). More specifically, for each current crowd and each favorite crowd of the user 16-1, the crowd comparison function 30 determines a degree of user overlap between the current crowd and the favorite crowd based on a comparison of the crowd hash value of the current crowd computed in step 606 and a crowd hash value stored for the favorite crowd in the crowd hash repository 32.

The crowd comparison function 30 then selects one or more of the relevant POIs to recommend to the user 16-1 based on the degrees of user overlap between the current crowds at the relevant POIs and the favorite crowd(s) of the user 16-1 (step 610). More specifically, in one embodiment, a relevant POI is selected to recommend to the user 16-1 if the degree of user overlap between a current crowd at the POI and at least one of the favorite crowds of the user 16-1 is greater than a predefined threshold (e.g., 75%). The crowd comparison function 30 then returns the recommended POI(s) to the mobile device 14-1 (step 612) where the crowd client 34-1 presents the recommended POI(s) to the user 16-1 (step 614).

FIG. 9 illustrates the operation of the crowd server 12 to anonymously compare historical crowds to one or more previously identified crowds of interest (e.g., favorite crowds) of a user according to one embodiment of the present disclosure. In this embodiment, the crowd hash computing function 28 computes and stores crowd hash values for crowds over time (step 700). These crowds are referred to herein as historical crowds. More specifically, in one embodiment, the crowd hash computing function 28 periodically queries the crowd formation function 26 for crowds of users formed at that time, computes crowd hash values for those crowds, and stores the crowd hash values in the crowd hash repository 32. The crowd hash computing function 28 may compute and store crowd hash values for all crowds or only those crowds in one or more defined geographic areas (e.g., crowds at one or more defined POIs or within one or more defined Areas of Interest (AOIs)).

At some point in time, the mobile device 14-1, and more specifically the crowd client 34-1 of the mobile device 14-1, sends a historical crowd comparison request to the crowd server 12 (step 702). In one embodiment, the historical crowd comparison request is for a current location of the user 16-1. In another embodiment, the historical crowd comparison request is for a POI or AOI defined or otherwise selected by the user 16-1. The historical crowd comparison request may be manually initiated by the user 16-1 or automatically initiated by the crowd client 34-1 in response to a triggering event.

In response to receiving the historical crowd comparison request, the crowd comparison function 30 obtains crowd hash values for historical crowds that are relevant to the historical crowd comparison request (step 704). In this embodiment, the crowd hash values for the historical crowds are stored in the crowd hash repository 32 along with the locations of the historical crowds at the time the crowd hash values were computed. As such, in one embodiment, the relevant historical crowds are historical crowds that were located at or near the current location of the user 16-1. In another embodiment, the relevant historical crowds are historical crowds that were located at a POI or within an AOI defined by the historical crowd comparison request. In addition, a time window may be defined for the historical crowd comparison request such that the relevant historical crowds are historical crowds for which the corresponding crowd hash values were computed during the defined time window. The time window may be system-defined or defined by the user 16-1 and included in the historical crowd comparison request. For example, the time window may be an absolute time window such as, for example, the last week, the last month, the last year, Jan. 1, 2010 through Mar. 12, 2010, or the like. As another example, the time window may be a reoccurring time window such as, for example, Fridays, weekdays from 10 AM-Noon, the first day of each month, or the like.

Next, for each historical crowd, the crowd comparison function 30 determines a degree of user overlap between the historical crowd and each of one or more favorite crowds previously identified by the user 16-1 using the process of FIG. 4 (step 706). Then, in this embodiment, the crowd comparison function 30 combines the degrees of overlap for the historical crowds to provide one or more combined degrees of user overlap (step 708). More specifically, for each favorite crowd of the user 16-1, the crowd comparison function 30 may combine (e.g., average) the degrees of user overlap between the historical crowds and the favorite crowd to provide a combined degree of user overlap for the favorite crowd. The combined degree of user overlap for each favorite crowd is then returned to the mobile device 14-1 (step 710). The crowd client 34-1 of the mobile device 14-1 then presents the combined degree of user overlap for each favorite crowd to the user 16-1 (step 712).

FIG. 10 illustrates the operation of the crowd server 12 to provide POI recommendations based on anonymous crowd comparisons according to another embodiment of the present disclosure. In this embodiment, the crowd hash computing function 28 computes and stores crowd hash values for crowds over time (step 800). These crowds are referred to herein as historical crowds. More specifically, in one embodiment, the crowd hash computing function 28 periodically queries the crowd formation function 26 for crowds of users formed at that time, computes crowd hash values for those crowds, and stores the crowd hash values in the crowd hash repository 32. The crowd hash computing function 28 may compute and store crowd hash values for all crowds or only those crowds in one or more defined geographic areas (e.g., crowds at one or more defined POIs or within one or more defined AOIs).

At some point in time, the mobile device 14-1, and more specifically the crowd client 34-1 of the mobile device 14-1, sends a POI recommendation request to the crowd server 12 (step 802). In response to receiving the POI recommendation request, the crowd hash computing function 28 obtains a list of relevant POIs (step 804). The list of relevant POIs may include POIs within a defined geographic region such as, for example, a defined geographic region in which the user 16-1 is currently located (e.g., within 10 miles from the current location of the user 16-1) or a geographic region defined by the user 16-1 and included in the POI recommendation request (e.g., Raleigh, N.C.). The crowd hash computing function 28 then obtains crowd hash values stored for historical crowds located at the relevant POIs from the crowd hash repository 32 (step 806). In one embodiment, a time window may be used such that the crowd hash values obtained for the relevant POIs are only those crowd hash values for historical crowds at the relevant POIs during a defined time window for the POI recommendation request. The time window for the historical crowd comparison request may be defined by the historical crowd comparison request or may be system-defined. For example, the time window may be an absolute time window such as, for example, the last week, the last month, the last year, Jan. 1, 2010 through Mar. 12, 2010, or the like. As another example, the time window may be a reoccurring time window such as, for example, Fridays, weekdays from 10 AM-Noon, the first day of each month, or the like.

Next, for each historical crowd for each relevant POI, the crowd comparison function 30 determines a degree of user overlap between the historical crowd and one or more favorite crowds previously identified by the user 16-1 using the process of FIG. 4 (step 808). More specifically, for each historical crowd and each favorite crowd of the user 16-1, the crowd comparison function 30 determines a degree of user overlap between the historical crowd and the favorite crowd based on a comparison of the crowd hash value of the historical crowd obtained from the crowd hash repository 32 and a crowd hash value stored for the favorite crowd in the crowd hash repository 32.

The crowd comparison function 30 then selects one or more of the relevant POIs to recommend to the user 16-1 based on the degrees of user overlap between the historical crowds at the relevant POIs and the favorite crowd(s) of the user 16-1 (step 810). More specifically, in one embodiment, a relevant POI is selected to recommend to the user 16-1 if the degree of user overlap between at least one historical crowd at the POI and at least one of the favorite crowds of the user 16-1 is greater than a predefined threshold (e.g., 75%). In another embodiment, for each relevant POI, the degrees of user overlap between historical crowds at the relevant POI for each favorite crowd of the user 16-1 are combined (e.g., averaged) to provide a combined degree of user overlap for the relevant POI for each favorite crowd of the user 16-1. A relevant POI may then be selected for recommendation to the user 16-1 if the combined degree of user overlap for the relevant POI for at least one of the favorite crowds of the user 16-1 is greater than a predefined threshold (e.g., 75%). The crowd comparison function 30 then returns the recommended POI(s) to the mobile device 14-1 (step 812) where the crowd client 34-1 presents the recommended POI(s) to the user 16-1 (step 814).

FIG. 11 illustrates the operation of the crowd server 12 to provide crowd monitoring at a desired POI based on anonymous crowd comparisons according to one embodiment of the present disclosure. First, the crowd server 12 receives a request to monitor crowds at a POI (step 900). The request may be from one of the mobile devices 14-1 through 14-N, the subscriber device 18, or the third-party service 22. The crowd hash computing function 28 then computes and stores crowd hash values for crowds at the POI over time (step 902). More specifically, in one embodiment, the crowd hash computing function 28 periodically queries the crowd formation function 26 for crowds at the POI, computes crowd hash values for the crowds at the POI, and stores the crowd hash values for the crowds in the crowd hash repository 32.

At some point, the crowd comparison function 30 compares the crowd hash values computed and stored for crowds at the POI over time to provide data characterizing crowd patterns at the POI (step 904). In other words, the crowd comparison function 30 characterizes crowd patterns at the POI based on comparisons of the crowd hash values computed and stored for crowds at the POI over time. More specifically, in one embodiment, for each of one or more reoccurring time windows, the crowd comparison function 30 determines a degree of user overlap between each crowd at the POI during the reoccurring time window and each other crowd at the POI during the reoccurring time window based on comparisons of corresponding crowd hash values. As a result, for each reoccurring time window, the crowd comparison function 30 provides a degree of user overlap between each pair of crowds at the POI during the reoccurring time window. Then, for each reoccurring time window, the degrees of user overlap computed for the pairs of crowds at the POI during the reoccurring time window may be combined (e.g., averaged) to provide a combined degree of user overlap for the reoccurring time window. The combined degree of user overlap for each of the reoccurring time windows may then be provided as data characterizing crowd patterns at the POI. Note that, as used herein, a reoccurring time window is a time window that periodically repeats itself. Some examples of a reoccurring time window are Friday (i.e., the day Friday repeats weekly), weekdays from 11 AM to 1 PM (repeats daily during the week and each week), March 19 (repeats yearly), or the like. Once characterization is complete, the data characterizing crowd patterns at the POI is returned to the requestor (step 906).

FIG. 12 is a block diagram of the crowd server 12 according to one embodiment of the present disclosure. As illustrated, the crowd server 12 includes a controller 40 connected to memory 42, one or more secondary storage devices 44, and a communication interface 46 by a bus 48 or similar mechanism. The controller 40 is a microprocessor, digital Application Specific Integrated Circuit (ASIC), Field Programmable Gate Array (FPGA), or the like. In this embodiment, the controller 40 is a microprocessor, and the crowd formation function 26, the crowd hash computing function 28, and the crowd comparison function 30 (FIG. 5) are implemented in software and stored in the memory 42 for execution by the controller 40. Further, the crowd hash repository 32 (FIG. 5) may be implemented in the one or more secondary storage devices 44. The one or more secondary storage devices 44 are digital data storage devices such as, for example, one or more hard disk drives. The communication interface 46 is a wired or wireless communication interface that communicatively couples the crowd server 12 to the network 24 (FIG. 5). For example, the communication interface 46 may be an Ethernet interface, local wireless interface such as a wireless interface operating according to one of the suite of IEEE 802.11 standards, or the like.

FIG. 13 is a block diagram of the mobile device 14-1 according to one embodiment of the present disclosure. This discussion is equally applicable to the other mobile devices 14-2 through 14-N. As illustrated, the mobile device 14-1 includes a controller 50 connected to memory 52, a communication interface 54, one or more user interface components 56, and the location determination function 36-1 by a bus 58 or similar mechanism. The controller 50 is a microprocessor, digital ASIC, FPGA, or the like. In this embodiment, the controller 50 is a microprocessor, and the crowd client 34-1 (FIG. 5) is implemented in software and stored in the memory 52 for execution by the controller 50. In this embodiment, the location determination function 36-1 is a hardware component such as, for example, a GPS receiver. The communication interface 54 is a wireless communication interface that communicatively couples the mobile device 14-1 to the network 24 (FIG. 5). For example, the communication interface 54 may be a local wireless interface such as a wireless interface operating according to one of the suite of IEEE 802.11 standards, a mobile communications interface such as a cellular telecommunications interface, or the like. The one or more user interface components 56 include, for example, a touchscreen, a display, one or more user input components (e.g., a keypad), a speaker, or the like, or any combination thereof.

FIG. 14 is a block diagram of the subscriber device 18 according to one embodiment of the present disclosure. As illustrated, the subscriber device 18 includes a controller 60 connected to memory 62, one or more secondary storage devices 64, a communication interface 66, and one or more user interface components 68 by a bus 70 or similar mechanism. The controller 60 is a microprocessor, digital ASIC, FPGA, or the like. In this embodiment, the controller 60 is a microprocessor, and the web browser 38 (FIG. 5) is implemented in software and stored in the memory 62 for execution by the controller 60. The one or more secondary storage devices 64 are digital storage devices such as, for example, one or more hard disk drives. The communication interface 66 is a wired or wireless communication interface that communicatively couples the subscriber device 18 to the network 24 (FIG. 5). For example, the communication interface 66 may be an Ethernet interface, local wireless interface such as a wireless interface operating according to one of the suite of IEEE 802.11 standards, a mobile communications interface such as a cellular telecommunications interface, or the like. The one or more user interface components 68 include, for example, a touchscreen, a display, one or more user input components (e.g., a keypad), a speaker, or the like, or any combination thereof.

FIG. 15 is a block diagram of a computing device 72 operating to host the third-party service 22 (FIG. 5) according to one embodiment of the present disclosure. The computing device 72 may be, for example, a physical server. As illustrated, the computing device 72 includes a controller 74 connected to memory 76, one or more secondary storage devices 78, a communication interface 80, and one or more user interface components 82 by a bus 84 or similar mechanism. The controller 74 is a microprocessor, digital ASIC, FPGA, or the like. In this embodiment, the controller 74 is a microprocessor, and the third-party service 22 is implemented in software and stored in the memory 76 for execution by the controller 74. The one or more secondary storage devices 78 are digital storage devices such as, for example, one or more hard disk drives. The communication interface 80 is a wired or wireless communication interface that communicatively couples the computing device 72 to the network 24 (FIG. 5). For example, the communication interface 80 may be an Ethernet interface, local wireless interface such as a wireless interface operating according to one of the suite of IEEE 802.11 standards, a mobile communications interface such as a cellular telecommunications interface, or the like. The one or more user interface components 82 include, for example, a touchscreen, a display, one or more user input components (e.g., a keypad), a speaker, or the like, or any combination thereof.

Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. 

What is claimed is:
 1. A computer-implemented method comprising: obtaining a hash value for a first user group, the hash value for the first user group comprising a plurality of component hash values generated for a plurality of two-user permutations for the first user group; obtaining a hash value for a second user group, the hash value for the second user group comprising a plurality of component hash values generated for a plurality of two-user permutations for the second user group; and determining a degree of user overlap between the first user group and the second user group based on a comparison of the hash value for the first user group and the hash value for the second user group.
 2. The method of claim 1 wherein: the plurality of two user-permutations for the first user group comprises all distinct two-user permutations for the first user group, and each of the plurality of component hash values for the first user group is a hash value computed for a corresponding one of the plurality of two-user permutations for the first user group based on a predetermined hash function; and the plurality of two user-permutations for the second user group comprises all distinct two-user permutations for the second user group, and each of the plurality of component hash values for the second user group is a hash value computed for a corresponding one of the plurality of two-user permutations for the second user group based on the predetermined hash function.
 3. The method of claim 1 wherein: the plurality of two user-permutations for the first user group comprises all distinct two-user permutations for the first user group other than those that include a requesting user associated with the hash value for the first user group, and each of the plurality of component hash values for the first user group is a hash value computed for a corresponding one of the plurality of two-user permutations for the first user group based on a predetermined hash function; and the plurality of two user-permutations for the second user group comprises all distinct two-user permutations for the second user group other than those that include a requesting user associated with the hash value for the second user group, and each of the plurality of component hash values for the second user group is a hash value computed for a corresponding one of the plurality of two-user permutations for the second user group based on the predetermined hash function.
 4. The method of claim 1 wherein obtaining the hash value for the first user group comprises: obtaining a list of users in the first user group; removing a requesting user that initiated a process for obtaining the hash value for the first user group from the list of users; sorting the list of users in the first user group to provide a sorted list of users; creating the plurality of two-user permutations for the first user group from the sorted list of users; computing a component hash value for each of the plurality of two-user permutations for the first user group to provide the plurality of component hash values for the first user group; and concatenating the plurality of component hash values to provide a concatenated hash value for the first user group.
 5. The method of claim 4 wherein obtaining the hash value for the first user group further comprises compressing the concatenated hash value for the first user group using a lossy compression algorithm to provide the hash value for the first user group.
 6. The method of claim 5 wherein compressing the concatenated hash value comprises removing every Nth bit, where N is greater than or equal to
 2. 7. The method of claim 5 wherein compressing the concatenated hash value comprises removing every other bit.
 8. The method of claim 4 wherein sorting the list of users in the first user group to provide the sorted list of users is such that the plurality of two-user permutations for the first user group are canonical with respect to other instances of the plurality of two-user permutations for other user groups including the second user group.
 9. The method of claim 1 wherein obtaining the hash value for the first user group comprises: obtaining a list of users in the first user group; sorting the list of users in the first user group to provide a sorted list of users; creating the plurality of two-user permutations for the first user group from the sorted list of users; computing a component hash value for each of the plurality of two-user permutations for the first user group to provide the plurality of component hash values for the first user group; and concatenating the plurality of component hash values to provide a concatenated hash value for the first user group.
 10. The method of claim 9 wherein obtaining the hash value for the first user group further comprises compressing the concatenated hash value for the first user group using a lossy compression algorithm to provide the hash value for the first user group.
 11. The method of claim 10 wherein compressing the concatenated hash value comprises removing every Nth bit, where N is greater than or equal to
 2. 12. The method of claim 10 wherein compressing the concatenated hash value comprises removing every other bit.
 13. The method of claim 9 wherein sorting the list of users in the first user group to provide the sorted list of users is such that the plurality of two-user permutations for the first user group are canonical with respect to other instances of the plurality of two-user permutations for other user groups including the second user group.
 14. The method of claim 1 wherein determining the degree of user overlap between the first user group and the second user group comprises: determining a number of matching component hash values between the plurality of component hash values for the first user group and the plurality of component hash values for the second user group; and determining a number of matching users in the first and second user groups based on the number of matching component hash values.
 15. The method of claim 14 wherein the number of matching component hash values corresponds to a number of matching two-user permutations between the plurality of two-user permutations for the first user group and the plurality of two-user permutations for the second user group.
 16. The method of claim 14 wherein determining the number of matching users in the first and second user groups based on the number of matching component hash values comprises determining the number of matching users in the first and second user groups based on: ${\frac{n!}{{2!}{\left( {n - 2} \right)!}} = {{number\_ of}{\_ matching}{\_ two}{\_ user}{\_ permutations}}},$ where number_of_matching_two_user_permutations is the number of matching component hash values and n is the number of matching users in the first and second user groups.
 17. The method of claim 14 wherein determining the degree of user overlap between the first user group and the second user group further comprises: determining a largest user group of the first and second user groups; and determining a percentage of user overlap based on a ratio of the number of matching users over a number of users in the largest user group of the first and second user groups.
 18. A server comprising: a communication interface communicatively coupling the server to a network; and a controller associated with the communication interface and adapted to: obtain a hash value for a first user group, the hash value for the first user group comprising a plurality of component hash values generated for a plurality of two-user permutations for the first user group; obtain a hash value for a second user group, the hash value for the second user group comprising a plurality of component hash values generated for a plurality of two-user permutations for the second user group; and determine a degree of user overlap between the first user group and the second user group based on a comparison of the hash value for the first user group and the hash value for the second user group.
 19. A computer-readable medium storing software for instructing a controller to: obtain a hash value for a first user group, the hash value for the first user group comprising a plurality of component hash values generated for a plurality of two-user permutations for the first user group; obtain a hash value for a second user group, the hash value for the second user group comprising a plurality of component hash values generated for a plurality of two-user permutations for the second user group; and determine a degree of user overlap between the first user group and the second user group based on a comparison of the hash value for the first user group and the hash value for the second user group.
 20. A computer-implemented method comprising: receiving a crowd hash value storage request from a requestor; obtaining a crowd in which the requestor is located at a time of receiving the crowd hash value storage request; computing a crowd hash value for the crowd in which the requestor is located, the crowd hash value comprising a plurality of component hash values computed for a plurality of two-user permutations for the crowd in which the requestor is located; and storing the crowd hash value as a crowd hash value for a crowd of interest of the requestor.
 21. The method of claim 20 further comprising, at some time after storing the crowd hash value for the crowd of interest of the requestor: receiving a current crowd comparison request from the requestor; obtaining a current crowd that is relevant to the current crowd comparison request; computing a crowd hash value for the current crowd, the crowd hash value for the current crowd comprising a plurality of component hash values computed for a plurality of two-user permutations for the current crowd; obtaining the crowd hash value of the crowd of interest of the requestor from storage; determining a degree of user overlap between the current crowd and the crowd of interest of the requestor based on a comparison of the hash value for the current crowd and the hash value for the crowd of interest of the requestor; and returning the degree of user overlap between the current crowd and the crowd of interest of the requestor to the requestor.
 22. The method of claim 20 further comprising, at some time after storing the crowd hash value for the crowd of interest of the requestor: receiving a Point of Interest (POI) recommendation request from a requestor; obtaining a plurality of relevant POIs that are relevant to the POI recommendation request; for each relevant POI of the plurality of relevant POIs: obtaining a current crowd at the relevant POI; computing a crowd hash value for the current crowd at the relevant POI, the crowd hash value comprising a plurality of component hash values computed for a plurality of two-user permutations for the current crowd at the relevant POI; and determining a degree of user overlap between the current crowd at the relevant POI and the crowd of interest of the requestor based on a comparison of the crowd hash value for the current crowd at the relevant POI and the crowd hash value for the crowd of interest of the requestor obtained from storage; selecting one or more recommended POIs from the plurality of relevant POIs based on the degree of user overlap between each current crowd at the plurality of relevant POIs and the crowd of interest of the requestor; and returning the one or more recommended POIs to the requestor.
 23. The method of claim 20 further comprising, at some time after storing the crowd hash value for the crowd of interest of the requestor: receiving a historical crowd comparison request from the requestor; obtaining one or more historical crowds that are relevant to the historical crowd comparison request; obtaining the crowd hash value of the crowd of interest of the requestor from storage; for each historical crowd of the one or more historical crowds: obtaining a crowd hash value for the historical crowd, the crowd hash value for the historical crowd being previously computed and stored and comprising a plurality of component hash values computed for a plurality of two-user permutations for the historical crowd; and determining a degree of user overlap between the historical crowd and the crowd of interest of the requestor based on a comparison of the hash value for the historical crowd and the hash value for the crowd of interest of the requestor; and returning, to the requestor, data reflecting the degree of user overlap between each of the one or more historical crowds and the crowd of interest of the requestor.
 24. The method of claim 23 further comprising: combining the degrees of user overlap between the one or more historical crowds and the crowd of interest of the requestor to provide a combined degree of user overlap; wherein returning the data comprises returning the combined degree of user overlap.
 25. The method of claim 20 further comprising, at some time after storing the crowd hash value for the crowd of interest of the requestor: receiving a Point of Interest (POI) recommendation request from a requestor; obtaining a plurality of relevant POIs that are relevant to the POI recommendation request; for each relevant POI of the plurality of relevant POIs: obtaining one or more historical crowds at the relevant POI; for each historical crowd of the one or more historical crowds at the relevant POI, computing a crowd hash value for the historical crowd at the relevant POI, the crowd hash value comprising a plurality of component hash values computed for a plurality of two-user permutations for the historical crowd at the relevant POI; and for each historical crowd of the one or more historical crowds at the relevant POI, determining a degree of user overlap between the historical crowd at the relevant POI and the crowd of interest of the requestor based on a comparison of the crowd hash value for the historical crowd at the relevant POI and the crowd hash value for the crowd of interest of the requestor obtained from storage; selecting one or more recommended POIs from the plurality of relevant POIs based on the degree of user overlap between each of the historical crowd at each of the plurality of relevant POIs and the crowd of interest of the requestor; and returning the one or more recommended POIs to the requestor.
 26. A server comprising: a communication interface communicatively coupling the server to a network; and a controller associated with the communication interface and adapted to: receive a crowd hash value storage request from a requestor via the communication interface; obtain a crowd in which the requestor is located at a time of receiving the crowd hash value storage request; compute a crowd hash value for the crowd in which the requestor is located, the crowd hash value comprising a plurality of component hash values computed for a plurality of two-user permutations for the crowd in which the requestor is located; and store the crowd hash value as a crowd hash value for a crowd of interest of the requestor.
 27. A computer-readable medium storing software for instructing a controller of a computing device to: receive a crowd hash value storage request from a requestor; obtain a crowd in which the requestor is located at a time of receiving the crowd hash value storage request; compute a crowd hash value for the crowd in which the requestor is located, the crowd hash value comprising a plurality of component hash values computed for a plurality of two-user permutations for the crowd in which the requestor is located; and store the crowd hash value as a crowd hash value for a crowd of interest of the requestor.
 28. A computer-implemented method comprising: receiving a request to monitor crowds at a Point of Interest (POI) from a requestor; computing a plurality of crowd hash values for a plurality of crowds at the POI over time, wherein, for each crowd hash value of the plurality of crowd hash values, the crowd hash value comprises a plurality of component hash values computed for a plurality of two-user permutations for a corresponding one of the plurality of crowds at the POI over time; comparing the plurality of crowd hash values for the plurality of crowds at the POI over time to one another to provide data characterizing crowd patterns at the POI; and returning the data characterizing crowd patterns at the POI to the requestor.
 29. The method of claim 28 wherein comparing the plurality of crowd hash values for the plurality of crowds at the POI over time to one another to provide the data characterizing crowd patterns at the POI comprises: identifying a subset of the plurality of crowds that were at the POI during a reoccurring time window; determining a combined degree of user overlap for the subset of the plurality of crowds based on a subset of the plurality of crowd hash values computed for the subset of the plurality of crowds; and including the combined degree of user overlap for the subset of the plurality of crowds in the data characterizing crowd patterns at the POI.
 30. The method of claim 28 wherein comparing the plurality of crowd hash values for the plurality of crowds at the POI over time to one another to provide the data characterizing crowd patterns at the POI comprises: identifying a subset of the plurality of crowds that were at the POI during a reoccurring time window; determining a degree of user overlap between each pair of crowds in the subset of the plurality of crowds that were at the POI during the reoccurring time window based on comparisons of crowd hash values in a subset of the plurality of crowd hash values for the subset of the plurality of crowds to one another; combining the degrees of user overlap for the pairs of crowds in the subset of the plurality of crowds that were at the POI during the reoccurring time window to provide a combined degree of user overlap for the reoccurring time window; and including the combined degree of user overlap for the reoccurring time window in the data characterizing crowd patterns at the POI.
 31. A server comprising: a communication interface communicatively coupling the server to a network; and a controller associated with the communication interface and adapted to: receive a request, via the communication interface, to monitor crowds at a Point of Interest (POI); compute a plurality of crowd hash values for a plurality of crowds at the POI over time, wherein, for each crowd hash value of the plurality of crowd hash values, the crowd hash value comprises a plurality of component hash values computed for a plurality of two-user permutations for a corresponding one of the plurality of crowds at the POI over time; compare the plurality of crowd hash values for the plurality of crowds at the POI over time to one another to provide data characterizing crowd patterns at the POI; and return the data characterizing crowd patterns at the POI to the requestor.
 32. A computer-readable medium storing software for instructing a controller of a computing device to: receive a request to monitor crowds at a Point of Interest (POI); compute a plurality of crowd hash values for a plurality of crowds at the POI over time, wherein, for each crowd hash value of the plurality of crowd hash values, the crowd hash value comprises a plurality of component hash values computed for a plurality of two-user permutations for a corresponding one of the plurality of crowds at the POI over time; compare the plurality of crowd hash values for the plurality of crowds at the POI over time to one another to provide data characterizing crowd patterns at the POI; and return the data characterizing crowd patterns at the POI to the requestor. 